Heretofore, cellulose acylate films have been used in photographic supports and various optical materials because of their good toughness and flame retardancy. In particular, recently they have become much used for optical transparent films for liquid-crystal display devices. As having high optical transparency and high optical isotropy, cellulose acylate films are excellent as optical materials for devices for polarization such as liquid-crystal display devices. Heretofore, they are used as protective films for polarizers and supports for optically-compensatory films capable of bettering the display images in the oblique direction (as viewing angle compensation).
A polarizer, a part of a liquid-crystal display device comprises a polarizing element and a protective film for the element attached to at least one side of the element. In general, a polarizing element is obtained by coloring an oriented polyvinyl alcohol (PVA) film with iodine or a dichroic dye. In many cases, a cellulose acylate film, especially a triacetyl cellulose film is used for the protective film for a polarizing element, as the film can be directly stuck to PVA. In attaching a protective film to a polarizing element, the surface of the protective film that is to be stuck to a polarizing element is subjected to surface treatment for hydrophilication for the purpose of bettering the adhesiveness of the film surface to a polarizing element. For the hydrophilication treatment, generally employed is alkali treatment, plasma treatment or corona treatment (for example, JP-A 2002-328224, JP-A 2000-356714). It is important that the protective film for a polarizing element has good optical isotropy, and the optical properties of the protective film for a polarizing element have significant influences on the properties of the polarizer that comprises the protective film.
Recently, liquid-crystal display devices have been much more required to have further improved viewing angle characteristics, and the optical transparent films for the protective films for polarizing elements and for the supports for optically-compensatory films are also required to have further increased optical isotropy. For the optical isotropy, it is important that the retardation, which is represented by the product of the birefringence and the thickness of an optical film, is small. In particular, for bettering the image display in the oblique direction, not only the retardation in the in-plane direction (Re) but also the retardation in the film thickness direction (Rth) must be reduced. Concretely, when the optical properties of an optical transparent film are evaluated, it is important that Re measured in the in-plane direction of the film is small and Re does not change even when measured in different angles.
Heretofore, cellulose acylate films having a reduced in-plane Re are known. However, cellulose acylate films of which the angle-dependent Re change is reduced, or that is, those having a reduced Rth are difficult to produce. Given that situation, optical transparent films having a reduced angle-dependent Re change have been proposed by the use of polycarbonate films or thermoplastic cyclo-olefin films in place of cellulose acylate films (for example, JP-A 2001-318233 and JP-A 2002-328233; commercial products of ZEONOR (by Nippon Zeon), ARTON (by JSR)). However, when these optical transparent films are used for protective films for polarizers, they are problematic in point of their adhesiveness to PVA as they are hydrophobic. Another problem with them is that the overall in-plane optical properties of the films are not uniform.
For solving the problems, it is strongly desired to improve the adhesiveness to PVA of optical transparent films of optical isotropy. Concretely, the optical anisotropy of cellulose acylate films is lowered, and more concretely, the in-plane Re of cellulose acylate films is reduced to almost zero, and the angle-dependent retardation thereof is reduced, or that is, Rth of the films is reduced to almost zero. As a result, the thus-improved films could be optical transparent films of optical isotropy, and their adhesiveness to PVA is desired to be further improved.
In producing cellulose acylate films, a compound serving as a plasticizer is generally added for the purpose of bettering the film-forming property of the film materials. Some plasticizers are disclosed, including, for example, phosphoric acid triesters such as triphenyl phosphate, biphenyldiphenyl phosphate, and phthalates (for example, Lecture of Plastic Materials, Vol. 17, by Nikkan Kogyo Shinbun, “Cellulose Resins”, p. 121 (1970)). Of those plasticizers, it is known that some are effective for lowering the optical anisotropy of cellulose acylate films. For example, specific fatty acid esters are disclosed (for example, JP-A 2001-247717). However, the effect of such known compounds for lowering the optical anisotropy of cellulose acylate films is not satisfactory.
Recent liquid-crystal display devices have become further required to have improved color display capability. For this, the optical transparent films for the protective films for polarizers and for the supports for optically-compensatory films must be so improved that not only their Re and Rth in the visible right region falling within a wavelength range of from 400 to 800 nm but also the wavelength-dependent Re and Rth change, or that is, the wavelength-dependent Re and Rth distribution of the films could be small.